Cleavage alcohols, sodium cyanoborohydride

With strong protic acids, the regiochemistry of the reductive cleavage reverses to give the benzyl ether of the less hindered alcohol. One of the few metal hydrides that can withstand the harshly acidic conditions of the reaction is sodium cyanoborohydride. When used in large excess, it will reduce benzylidene acetals in the presence of anhydrous HG [Scheme 3.61] -or trifluoro-methanesulfonic acid. The excess is required because the sodium cyanoborohydride is consumed under the reaction conditions at an appreciable rate. More convenient reaction conditions were described by DeNinno and co-workers in which the benzylidene acetal is reductively cleaved using triethylsilane in the presence of trifluoroacetic acid [Scheme 3.62]. ... 

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